When a polarizing film is used under such conditions that it will be exposed to the air, or will be in a display in which a liquid crystal is used, it is necessary to provide a protective layer on the surface of the polarizing film in order to protect it from scratches, moisture or corrosion due to chemicals. Glass and plastic materials have been used in the preparation of such protective layers. These materials, however, suffer from various disadvantages. In the case of a glass protective layer, the glass layer is fairly thick for adequate strength and is heavy in weight. Plastic materials now in practical use are cellulose-based and acryl-based resins. Protective layers made of such materials are inferior in shape, dimensional stability, moisture resistance, and heat resistance, and are particularly unsuitable for outdoor uses.
An attempt has been made to use a film of polyethylene terephthalate (hereinafter sometimes referred to as "PET"), a kind of polyester, which has been longitudinally, transversely or biaxially stretched at a stretching ratio of, for example, 3 to 4. This stretched PET film shows a high corrosion resistance against chemicals and is superior in heat resistance and moisture resistance, but optical anisotropy is formed in the film by the action of stretching, causing birefringence. Therefore, when an object is seen through a polarizing plate protected by such a PET film, colored fringes due to the interference of light are sometimes formed on the surface of the PET protective layer. Thus, this type of PET film is not suitable for use as a protective layer for a polarizing plate as used in a display in which a liquid crystal is used.